Photo-responsive devices, in particular x-ray detectors are a key component for medical radiology where digital radiography becomes increasingly important. Digital radiography offers the potential of improved image quality and provides at the same time opportunities in medical image management, computer-aided diagnosis and teleradiology. Image quality depends critically on the precise and accurate detection of the x-ray beam transmitted by the patient and hence on the performance of the x-ray detector. Key parameters for the x-ray detector are spatial resolution, uniformity of response, contrast sensitivity, dynamic range, acquisition speed and frame rate.
In the field of medical applications flat dynamic x-ray detectors (FDXD) play an important role. Flat detectors are used by several manufacturers of commercially available x-ray devices for medical diagnostics. At present two different technologies for flat detectors coexist. One type of detector relies on so-called indirect conversion of x-rays, i.e. an absorbed x-ray quantum is converted into visible light and subsequently into an electrical signal, which is necessary for the digital processing of the captured image. Another type of detector relies on the direct conversion of x-rays, i.e. an x-ray quantum is directly converted into an electrical signal. In general, direct conversion detectors provide a higher spatial resolution and a higher signal-to-noise ration compared to indirect conversion detectors.
Flat direct conversion detectors comprise an absorption layer of a semi conducting material having a large band gap. In the absorption layer an absorbed x-ray quantum is converted into an electrical charge. Typically, the absorption or conversion layer is made of selenium, lead iodide, mercury iodide, lead oxide, cadmium telluride, cadmium zinc telluride. In general, the conversion layer is directly deposited on a matrix of thin film electronic circuitry. The following description of the prior art and the invention is focused on the use of lead oxide (PbO) as conversion layer
In direct conversion materials like the ones listed above, an electrical signal is generated if charge carriers, i.e. holes or electrons, move in the electrostatic field in the conversion layer. However, the conversion layer contains electrical defects like crystallographic defects, impurities etc. capable to capture free electric charge carriers. Captured charge carriers are also denominated as “localised” charge carriers. Localised charge carriers are lost for the generation of an electrical output signal of the detector. But localised charge carriers may become movable again, i.e. they become “delocalised”. Delocalised charge carriers contribute again to the electrical output signal of the detector. The principle of this process is the same for electrons (negative charge carriers) and holes (positive charge carriers). In thermodynamic equilibrium the average number of charge carriers becoming localised and delocalised is equal. Since the localisation and delocalisation process, respectively, requires some time it is obvious that the localisation and delocalisation of charge carriers has an impact on the dynamic response time of an x-ray detector.
Specifically in x-ray applications in which the detector is moving relative to an imaged object, the response time is critical for the resolution of transitions between areas of different physical properties, e.g. soft tissue and a bone of a patient. The transition is visible in an image as a dark to bright contrast and the sharpness of the image depends on the response time of the x-ray detector. Examples for such applications are volume imaging and the detection of a contrast agent flowing through vessels.
In practice, a long response time entails dynamic artefacts, which occur e.g. in detectors comprising direct conversion materials. Disturbing residual signals after the termination of an x-ray irradiation as well as a delayed signal rise at the beginning of an irradiation have been observed. The dynamic artefacts are visible as disturbing after images, in particular in dynamic imaging processes with a high repetition rate like volume imaging or fluoroscopy applications. Dynamic artefacts also occur in imaging moving objects, as e.g. a beating heart; likewise a respiration movement of a patient can cause dynamic artefacts.
Frequently, the reason for the mentioned dynamic artefacts is the relatively slow filling and emptying of electrical defects with charge carriers in the conversion layer.
Electrically active defects may be created by a discontinuity of the electronic properties in the growth direction of the conversion layer. Specifically the semi conducting material lead oxide (PbO) exists in two different crystallographic phases, namely tetragonal PbO (red lead oxide) and orthorhombic PbO (yellow lead oxide). In manufacturing of lead oxide layers according to conventional methods at first always a seeding layer of yellow orthorhombic lead oxide grows on the surface of the substrate. After a seeding layer of a few μm thickness has grown, the lead oxide continues to grow as red tetragonal lead oxide. Since the yellow orthorhombic PbO has a band gap of 2.7 eV and the red tetragonal PbO has a band gap of 1.9 eV there is a discontinuity present at the interface between the two different crystallographic phases of PbO. The band gap discontinuity is a significant disturbance of the electronic structure of the complete layer and represents electrical defects capable to localise free charge carriers. As it has been described above localised charge carriers can be delocalised again with a certain delay in time and entail disturbing after images.
U.S. Pat. No. 3,444,412 describes the manufacturing of a photo-responsive device having a photo-sensitive lead oxide (PbO) layer composed of sub layers of different conductivity types. The growth of the layer begins on the substrate with n-type PbO followed by a thick layer of intrinsic PbO. The final surface layer is grown from PbO to which thallium oxide has been added. Thallium oxide acts as a p-former for the PbO. The different starting materials are evaporated from the same crucible which is charged at the beginning only with PbO and subsequently recharged with a mixture of PbO and thallium oxide for growing the surface layer. Instead of thallium oxide a compound of a different element acting as a p-former for PbO or such an element itself are also disclosed. E.g. it is proposed to add PbF2 to the PbO though thallium oxide is preferred. The crystallographic and electronic discontinuity inside the PbO layer next to the substrate is not addressed.